In this project we wish to define parameters which determine the capacity of lymphoid cells and tumor cells to serve as stimulators in the mixed lymphocyte reaction and for the generation of cytotoxic T cells. The turnover and shedding rates of membrane alloantigens are being analyzed at the molecular level, and cellular processes controlling metabolism of these antigens are being studied. Factors produced by cytotoxic T cells which suppress specifically the immune performance of sensitized T cells are being isolated and characterized.